Torque Converter

What torque measures and why units differ

Torque is the rotational equivalent of force. It equals force multiplied by the perpendicular distance from the pivot point: T = F × d. A force of 10 N applied at 0.5 m from a pivot generates 5 N·m of torque. The SI unit is the newton-metre (N·m), which is dimensionally equivalent to a joule but used for torque rather than energy to avoid confusion.

In US automotive specifications, torque is typically quoted in pound-feet (lb·ft). This is the torque produced by a one-pound force acting at one foot from the pivot. One lb·ft equals approximately 1.35582 N·m. A car engine with 300 lb·ft of peak torque produces about 407 N·m. European and Japanese car manufacturers publish torque in N·m, which is why the same engine may be listed at "300 ft·lb" in a US brochure and "407 Nm" in a European one.

Pound-inches (lb·in) are used for smaller fasteners and precision applications where lb·ft would produce unwieldy decimal values. Torque specs for electronics chassis screws, bicycle components, and small engine assembly are often in lb·in or in-lb. One lb·ft equals 12 lb·in.

Kilogram-force metres (kgf·m or kg·m) appear in older engineering literature, particularly from Japan and Eastern Europe. One kgf·m equals 9.80665 N·m, because it uses kilogram-force (the force exerted by one kilogram under standard gravity) rather than newtons. Modern standards prefer N·m, but kgf·m values still appear in legacy technical manuals.

Torque wrenches and fastener specifications

Torque wrenches apply a controlled amount of tightening force to threaded fasteners. Under-tightening leads to loose joints; over-tightening can stretch or snap the fastener, strip threads, or warp gaskets. Engine head bolts, wheel lug nuts, and suspension components all have published torque specifications that must be followed precisely.

A cylinder head bolt torqued to 80 N·m requires a torque wrench calibrated in N·m. If the only wrench available reads in lb·ft, the mechanic must convert: 80 N·m ÷ 1.35582 = approximately 59 lb·ft. Getting this wrong by even 10% can cause head gasket failure or thread damage. Accurate unit conversion is a safety issue in these applications.

Frequently asked questions